Wine making is a complex blend of art, science and luck. Wine making is a very old process. For about 5,000 years man has used grapes to make wines by fermentation. Wine as used herein is defined as a liquid made by the partial or complete fermentation of the juice of grapes, and other fruits or berries. Grapes are the only fruit with a sufficiently high natural level of sugar and have the proper balance of acid and nutrients to sustain natural fermentation to dryness with stable results. Other fruits and berries may be fermented but usually require the addition of sugar, acid or various yeast nutrients to avoid spoiling.
Although there have been improvements in various aspects of the wine making process over the years, these improvements have been primarily in the equipment used in the processing of the grapes. The basic reaction by which grapes are transformed into wine remains unchanged. Typically, grapes are crushed to release the juice into a fermentation vessel. When the fermentation is complete the wine is pressed to separate the liquid from the stems, skin, pips and pulp. Wine is then stored to age and clarify.
Ripe grapes naturally have yeast cells residing on their surface that aid and abet the reaction of grapes into wine. When the yeast comes into contact with grape juice, the yeast begins to feed on the juice. The yeast contains an enzyme, zymase, that converts sugar in the grape juice to alcohol and carbon dioxide, as well as releasing heat. The reaction continues naturally until the sugar has been converted or the yeast dies off or weakens. The formula for the creation of wine is as followsC6H12O6→Zymase→2C2H5OH+2CO2 
Fermentation will usually take about three weeks. During the first few days of the process frequently called aerobic fermentation, the reaction usually produces more yeast through reproduction of the yeast cells. This first step is followed by anaerobic fermentation which produces the most alcohol. Fermentation may be permitted to continue until there is no residual sugar or the reaction may be terminated at some point during the process to vary the level of sweetness. The reaction is usually terminated by killing or removing the yeast cells. This may be accomplished by adding alcohol to raise the level to 15% or more, adding sulfur dioxide or sorbate (sorbic acid), by filtering through a sterile filter or by chilling the must and filtering out the yeast cells.
The color of the wine comes from contact by the grape juice which is clear, with the skin of the grapes. The more color the skin has plus the amount of time the juice is in contact with the skin increases the color of the wine. Different steps in the making of the wine can cause variations in the taste and bouquet of the wine. for example, juice separated from the must before pressing usually has less bitterness and oxidation. This is called “free run.” The leftover skins, pips and pulp is called the marc and usually has a lower fixed acidity but a higher volatile acidity as well as higher tannin than the free run. The speed and pressure of the press may also affect the wine. Too much pressure in the press may cause bitter tannins to leach from the seeds.
There is also a second fermentation that occurs in most wine making. This is called malolactic fermentation. In this type of fermentation, bacteria i.e., lactobacillus, converts some of the malic acid naturally present in grapes into lactic acid along with the resultant by product of carbon dioxide. Malolactic fermentation usually has the effect of softening the wine i.e., taking some of the edge off the wine.
Acids are a natural component of wine. However, if a wine is too low in acid the wine tastes too flat and dull. If the wine has too great an amount of acid the wine tastes too tart and sour. as a result, the wine maker frequently manipulates the acidity in the wine. The principal acids formed in grapes and therefore in wine are, tartaric acid, potassium hydrogen tartrate, malic acid and potassium hydrogen malate. The relative amounts of acid depend on the grape variety used to make the wine. In addition, the growing temperature of the grapes can also affect the amount of acidity in a wine. For example, the grapes grown in the Chablis region of France have a high acid content because the growing climate is relatively cool. On the other hand, the grapes grown in California's Napa Valley have a lower acidity because of the higher temperatures that these grapes grow at. Conversely, the higher the growing temperature, the higher the sugar content in the grape juice. Thus, the Napa Valley grapes have low acidity and higher sugar content than the Chablis grapes which are grown at a lower temperature.
Because of all the variations in the types of grapes, the region where the grapes are grown, the wine making process, the weather during a particular growing season and other factors there can be enormous variations in the taste of wines even made from the same type of grape. Some additional factors besides the foregoing that have a significant bearing on the quality of the final product include whether the barrels for the cellaring are made from French Oak, American Oak or some other material. Transport conditions and temperatures maintained during the distribution process are also important. For example, the length of time in transit in hot trucks can vary the quality of the wine compared to shipment in cooler conditions and for shorter periods of time. The length of time in a warehouse or on a retail shelf or exposure to U.V. light can introduce variations in the taste of a wine that are not always beneficial to the wine. In addition, because individual taste buds vary, the same wine may taste different to two different people. Thus a wine that tastes smooth to one person may taste sharper to another. In addition, some wine may be more bitter tasting than others.